This invention relates, in general, to an oxide formation method, and more particularly to a ramped oxide formation method
Prior art methods of forming high quality oxide layers have generally encountered many problems. In memory devices employing Fowler-Nordheim tunneling that employ the oxidation of polysilicon gates, it is desirable to keep temperatures in the range of approximately 900 to 1000 degrees centigrade to preserve endurance characteristics of the device. If the oxidation occurs at a higher temperature (1050 to 1100 degrees centigrade), the tunnel oxide is often degraded. If lower temperatures are employed, the oxide formed on the polysilicon gate is generally of a low quality because the poor grain structure results in a rough oxide surface (asperities).
Prior art oxidation methods have included dilute oxidation throughout the entire cycle wherein oxygen is diluted with an inert gas. Generally, dilute oxidation methods are relatively slow because it takes longer for oxygen to reach the polysilicon/oxide interface as the oxide becomes thicker. Additionally, the temperature constraints mentioned above are prevalent. Two step oxidations wherein a dilute oxygen flow is initially employed and then a substantially higher flow is later employed are also commonly used. However, if the low oxygen flow is maintained too long, there will be oxygen starvation (dangling bonds) at the polysilicon/oxide interface. Nitrogen anneals often followed by short bursts of oxygen and hydrogen anneals have been employed to remedy the oxygen starvation problem at the polysilicon/oxide interface and although these anneals are somewhat successful, they are difficult to control and still leave a good deal of unreacted polysilicon at the interface.